Also, I apologize in advance. I just noticed that you have a 75 gal tank with a 20 gal sump. That system is way to small for the skimmer you have. Considering water displacement, you probably have around 80 gal of water in the tank. The skimmer you have is made for much larger tanks. It can work on a 100 tank with HEAVY load.
Here is a great support article on why skimmers do not perform consistently. In your case, your skimmer is way too big for the tank you have...
http://www.coralvue.com/support/676043-Little-to-no-foam-production
Kind regards,
Carlos Chacon
CoralVue